Anisotropy Spectroscopy Research Articles

Chiroptical characterization tools for asymmetric small molecules - experimental and computational approaches for electronic circular dichroism (ECD) and anisotropy spectroscopy.

Synchrotron radiation electronic circular dichroism (SRECD) and anisotropy spectroscopy for both enantiomers of a group of small non-planar chiral molecules are reported here. The experimental SRECD spectra are compared to computational ECD spectra generated using time-dependent density functional theory and a thermal averaging over relevant molecular configurations. The combination of these experimental and computational characterization methodologies for such molecules enables the prediction and understanding of the spectral behavior of other small molecules, in addition to chiroptically characterizing members of the mandelic acid family substructure. Enantiomeric purity of samples can be evaluated in comparison with these spectra and the extent of photolytic enantioinduction can also be predicted using these experimental/calculated SRECD and anisotropy spectra.

Excitation/detection energy controlled anisotropy dynamics in asymmetrically cyano substituted tri-podal molecules.

In the present work, the photophysical properties of two series of asymmetrical tri-podal molecules are studied, in order to determine the dependence of energy localization/delocalization phenomena on excitation and detection wavelength, by means of steady state, femtosecond time-resolved fluorescence and anisotropy spectroscopy. The molecules bear triphenylamine as an electron donating core and an acetylenic or olefinic π-conjugated bridge. At the periphery, they are substituted by no, one, two or three -CN groups used as electron acceptors. Thus, the compounds with only one or two -CN groups are asymmetrically substituted. As a comparison, the photophysics of their dipolar and quadrupolar analogues is also presented. The steady state absorption spectra of the asymmetrical tri-podal compounds exhibit a broadening and a low energy shoulder due to the splitting of the excited states. The fluorescence spectra are more red-shifted in the tri-podal molecules with a single -CN group, providing the first evidence of its mostly dipolar nature. Time-resolved anisotropy measurements by using different excitation and detection wavelengths provide clear evidence that the asymmetrical tri-podal molecules with one or two -CN groups behave like octupolar molecules upon high-energy excitation (the initial anisotropy is found 0.1-0.15), while upon low-energy excitation they reveal a behavior expected for linear dipolar or V-shaped quadrupolar molecules (the initial anisotropy is very close to 0.4 and 0.17, respectively). The symmetrical tri-podal compounds with no or three cyano groups, exhibit an anisotropy depolarization time of 2.5 ps attributed to energy hopping. The amplitude of this energy hopping component is wavelength dependent and increases as the excitation is shifted towards the long wavelength edge.

Generating laser-pulse enantiomers.

We present an optical setup capable of mirroring an arbitrary, potentially time-varying, polarization state of an ultrashort laser pulse. The incident beam is split up in two and the polarization of one beam is mirrored by reflection off a mirror in normal incidence. Afterwards, both beams are recombined in time and space such that two collinear ultrashort laser pulses with mutually mirrored polarization, i.e., laser-pulse enantiomers, leave the setup. We employ the Jones formalism to describe the function of the setup and analyze the influence of alignment errors before describing the experimental implementation and alignment protocol. Since no wave plates are utilized, broadband pulses in a large wavelength range can be processed. In particular, we show that the setup outperforms broadband achromatic wave plates. Furthermore, since the two beams travel separately through the optical system they can be blocked independently. This opens the possibility for circular dichroism, ellipsometry, and anisotropy spectroscopy with shot-to-shot chopping and detection schemes as well as chiral coherent control applications.

Residual electric fields of InGaAs/AlAs/AlAsSb (001) coupled double quantum wells structures assessed by photoreflectance anisotropy

We report on photoreflectance anisotropy (PRA) spectroscopy of InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQWs) with extremely thin coupling AlAs barriers grown by molecular beam epitaxy (MBE), with no intentional doping. By probing the in-plane interfacial optical anisotropies (OAs), it is shown that PRA spectroscopy has the ability to detect and distinguish semiconductor layers with quantum dimensions, as the anisotropic photoreflectance (PR) signal stems entirely from buried quantum wells (QWs). In order to account for the experimental PRA spectra, a theoretical model at k = 0, based on a linear electro-optic effect through a piezoelectric shear strain, has been employed to quantify the internal electric fields across the QWs. The dimensionalities of the PR lineshapes were tested by using reciprocal (Fourier) space analysis. Such a complementary test is used in order to correctly employ the PRA model developed here.

Characterization of Si3N4/Si(111) thin films by reflectance difference spectroscopy

Si3N4 has become an important material with great technological and scientific interests. The lattice symmetry and the crystallinity quality of Si3N4 thin films are fundamental parameters that must be determined for different applications. In order to evaluate the properties of Si3N4 films, we used reflectance difference spectroscopy/reflectance anisotropy spectroscopy (RDS/RAS) to measure the optical anisotropy of Si3N4 thin films (1–2 nm) grown by nitridation of two different Si(111) substrates, one with a 4.2° miscut off towards the direction and another one with a nonintentional miscut. We demonstrate that, by modifying the measurement optical setup, we could increase the RD sensitivity and clearly display the optical response corresponding to the hexagonal symmetry of the Si3N4 thin layer. Our results are in good agreement with reflection high energy electron diffraction (RHEED) measurements for both misoriented and oriented substrates.

Understanding photochirogenesis: solvent effects on circular dichroism and anisotropy spectroscopy.

The basic units that constitute essential biopolymers (proteins and nucleic acids) are enantiomerically biased. Proteins are constructed from L-amino acids and nucleic acids possess a backbone composed exclusively of D-sugars. Photochirogenesis has been postulated to be the source of this homochirality of biomolecules: Asymmetric photochemical reactions were catalyzed by circularly polarized light (cpl) in interstellar environments and generated the first chiral prebiotic precursors. Enantiomers absorb cpl differently and this difference can dictate the kinetics of asymmetric photochemical reactions. These differences in absorption can be studied using circular dichroism (CD) and anisotropy spectroscopy. Rather than measuring the CD spectrum alone, the anisotropy factor g is recorded (CD divided by absorption). This factor g is directly related to the maximum achievable enantiomeric excess. We now report on the substantial influence of solvent and molecular surroundings on CD and anisotropy spectroscopy. This shows for the first time that CD and anisotropy signals depend just as much on the molecular surroundings of a molecule as on the nature of the molecule itself. CD and g spectra of amino acids in different solvents and in the solid state are presented here and the influence of these different surroundings on the spectra is discussed.

Diffusivity in water and fluorescence properties of organic nanoparticles produced in flames

Rotational and translational diffusion properties in water of nano organic carbon (NOC) particles collected from premixed laminar ethylene/air flames have been investigated using both time resolved fluorescence polarisation anisotropy (TRFPA) and fluorescence correlation spectroscopy (FCS). Insight into the NOC sizes, structures and rigidity has been gained through diffusion properties exploiting their fluorescence in different spectral ranges. The TRFPA measurements revealed the presence of two classes of particles. The first composed of particles with a mean size of 1.5 nm which absorb and fluoresce in the UV, and a second class composed of slightly larger particles, about 2.2 nm, which absorb and fluoresce in the visible and were also detected by FCS. From FCS measurements particle concentration and fluorescence quantum yield have been evaluated.

Reactor-scale profile of group-V composition of InGaAsP studied by fluid dynamics simulation and in situ analysis of surface kinetics

Reactor-scale simulation of metalorganic vapor phase epitaxy (MOVPE) by computational fluid dynamics is now widely attempted, but it is still difficult to predict group V composition precisely for InGaAsP. In this study, desorption of As and P from the surface of GaAs, InAs, GaP, InP, InGaAs and InGaP was measured around growth temperatures using in situ reflectance anisotropy (RA) spectroscopy. Time transient curves of RA were measured when group V supply was turned on/off and the rate constants of the desorption of As and P were measured assuming first-order kinetics. These data were incorporated into the simulation to obtain the composition of InGaAsP. The simulated distribution of group V composition showed better agreement with experimental data than the results using previous kinetic data.

Fast energy transfer within a self-assembled cyclic porphyrin tetramer

The structure of a cyclic self-assembled tetramer of an asymmetric meso-ethynylpyridyl-functionalized Zn(II)-porphyrin was established by solution-phase X-ray scattering and diffraction; femtosecond transient absorption and anisotropy spectroscopies were used to (a) observe rapid energy transfer (3.8 ps(-1)) between porphyrin subunits and (b) establish that the transfer occurs between adjacent units.

GaNAs(001) surface phases under growing condition

GaNAs∕GaAs(001) growth surfaces were investigated by reflectance difference (RD)∕anisotropy spectroscopy. In situ monitoring was carried out in a plasma-assisted molecular-beam epitaxy system during growth. In the growth temperature range of our experiment, the reflection high-energy electron diffraction pattern always showed a (2×4) reconstruction pattern, however, we observed three types of RD spectra. It was suggested that the GaNAs growth surface has three phases because RD spectra properly reflect surface electronic states. It is revealed that the phases are independent from nitrogen concentration, but dependent upon growth temperature and As4∕Ga flux ratio. In this article, we show the results of the RD observation, present a phase diagram, and discuss possible GaNAs growth surfaces.

Optical anisotropy and surface morphology of InGaAs lattice-mismatched with GaAs(001)

Abstract We investigated surface morphology and optical anisotropy of strained InGaAs films grown on GaAs(0 0 1) substrate using atomic force microscopy (AFM) and reflectance difference/reflectance anisotropy spectroscopy (RDS/RAS). High temperature (HT)-grown samples were found to have a rippled surface structure, however for films grown using a low temperature (LT) growth technique, the surface morphology was significantly improved, without the ripple structure seen on the HT samples. Furthermore, ex situ RD spectra of LT-grown samples showed notable peaks near the critical energies of band structure originated from bulk electronic transitions.

In Situ Optical Techniques for Monitoring the Formation of Nanostructures

Various optical techniques have been developed to meet new challenges in epitaxial growth. The primary parameters needed for growth control, layer thicknesses, and composition strongly influence the formation of nanostructures on surfaces. These two parameters can be obtained with spectroscopic ellipsometry. In this article, we discuss a similiar technique, reflectance difference/ anisotropy spectroscopy, which is capable of monitoring in situ the surface stress occuring from surface reconstructions, e.g. dimerization. Using reflectance difference spectroscopy in situ the size and shape of epitaxially grown self-assembling Mn-based nanostructures was reproducibly achieved by tracing the formation process. A variety of well controlled strain-induced island morphologies was obtained with the deposition of semiconducting materials on different Mn surfaces.

Syntheses of Phencyclone Analogues. Applications for NMR Studies of Hindered Rotations and Magnetic Anisotropy in Crowded Diels?Alder Adducts

Undergraduate organic chemistry special laboratory projects based upon the synthesis of phencyclone, 1 (a potent Diels-Alder diene), and the preparation of a series of highly hindered Diels-Alder adducts of the phencyclone, were described earlier. Details of the synthesis of an analogue of 1 are presented here as an extension of these projects. The analogue, 3,6-dibromophencyclone, 2, and adducts from a wide range of dienophiles, can be prepared by undergraduate organic chemistry students. These adducts (from 1 or 2) are eminently suitable for student characterization by NMR to examine aspects of hindered rotation, magnetic anisotropy, and dynamic NMR spectroscopy, using modern one- and two-dimensional multinuclear methods with a medium-field instrument (7 T), to observe 1H, 13C, and 19F. Use of 2 effectively doubles the range of potential target compounds for students. The Diels-Alder adducts (and their precursors) have been studied by molecular modeling methods. This present paper describes the reaction of 2 with 4, the dienophile N-(4-dimethylamino-3,5-dinitrophenyl)maleimide (“Tuppy’s maleimide”), to form the adduct 5. Compound 5 has been well-characterized by 1D and 2D 1H and 13C NMR, and is illustrative of the wide range of adducts that can be made from 2 by students. The structure of 5, as determined by geometry optimization at the semiempirical (AM-1) level, is included here.

Investigation and Control of MOVPE Growth by Combined Spectroscopic Ellipsometry and Reflectance-Difference Spectroscopy

The detailed characterization of epitaxial growth by metal organic vapor phase epitaxy (MOVPE) and its closed-loop feedback control at the sample level require a combination of thin-film, near-surface, and surface-sensitive techniques to determine layer thicknesses and compositions, the composition of the most recently deposited material, and surface chemistry, respectively. These data can be obtained nondestructively by spectroscopic ellipsometry (SE) and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). We describe the first unified optical system, basically a rotating-polarizer ellipsometer (RPE) integrated within a modified commercial rotating-sample MOVPE reactor, that performs both SE and RDS simultaneously in a single optical path. We provide examples of its use, showing in particular that GaP can intermix with Si during the initial stages of heteroepitaxy, and demonstrating sample-driven closed-loop feedback control of epitaxy through the fully automatic deposition of an InxGa1—xP parabolic quantum well. These results illustrate capabilities of the presented configuration and its potential for future use.

In situ monitoring of MOVPE growth by combined spectroscopic ellipsometry and reflectance-difference spectroscopy

Comprehensive characterization of epitaxial growth by metal organic vapor phase epitaxy (MOVPE) requires a combination of thin-film, near-surface-, and surface-sensitive techniques to determine layer thicknesses and compositions, composition of the most recently deposited material, and surface chemistry, respectively. These data can be obtained non-destructively by spectroscopic ellipsometry (SE) and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Here we describe the first unified optical system, basically a rotating-polarizer ellipsometer (RPE) integrated into a modified commercial rotating-sample MOVPE reactor, that performs both SE and RDS simultaneously with a single optical path. Data are obtained in parallel from 240 to 840 nm with a high-speed 16-bit photodiode array (PDA) at a repetition rate greater than 2 Hz and a precision of ±0.0001. We provide examples of its use, and show in particular that GaP intermixes with Si during the initial stages of heteroepitaxy. Capabilities of the presented configuration and its potential for future investigations are discussed.

A Reflection High Energy Electron Diffraction–Reflectance Anisotropy Spectroscopy Study of Silicon Growth Dynamics During Gas Source Molecular Beam Epitaxy from Silanes

Molecular beam epitaxy (MBE) provides an ideal experimental vehicle for the in situ study of thin film growth dynamics. By using a combination of reflection high energy electron diffraction (RHEED) and reflectance anisotropy (difference) spectroscopy [RA(D)S], it is possible to separate morphological (long range order) and local electronic structure effects, which we demonstrate with the growth of silicon films from disilane ( Si 2 H 6) on Si(001) (2 × 1)+(1 × 2) reconstructed surfaces. The rate-limiting step in Si growth from both monosilane ( SiH 4) and disilane is the desorption of molecular hydrogen and we have found using RAS that, over a significant range of temperature and coverage, hydrogen desorption follows zeroth order kinetics as the result of a step-mediated process. Finally, we show how this influences the growth rate on substrates of differing degrees of vicinality.

Real-Time Surface and Near-Surface Optical Diagnostics for Epitaxial Growth

AbstractVarious optical diagnostics are being developed to meet new challenges in semiconductor epitaxy. New data-analysis algorithms allow near-surface alloy compositions and interface properties to be obtained by kinetic ellipsometry (KE) without any knowledge of the underlying sample structure. New information about growth is being obtained by surface-oriented probes such as laser light scattering (LLS), surface photoabsorption (SPA), anrd reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Examples are provided and likely directions of further progress discussed.